Modern vehicles typically employ a starting device configured to transfer rotating power from a prime mover, i.e., a power-source, such as an internal combustion engine or electric motor, to a rotating driven load, such as a given vehicle's wheel(s). Such a starting device is frequently arranged between the power-source and a vehicle's transmission and permits power-source torque to be used for moving or starting the vehicle from rest, or, when the vehicle is stationary, permits the power-source to be separated from the driven wheel(s).
In vehicles with parallel-shaft manually and automatically controlled transmissions, such a starting clutch is frequently a mechanically engageable clutch capable of providing a rigid connection between the power-source and the driven wheel(s). Such mechanical clutches typically employ selectively clamped friction disc(s) to affect the connection between the power-source and the transmission.
In vehicles with planetary automatic transmissions such a starting device is frequently a fluid coupling, a.k.a., a torque converter. A key characteristic of torque converters is their ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. A torque converter can also be equipped with a selectively locking mechanism to rigidly connect the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.